Wireless automatic meter reading systems are well known. Typically, each utility meter is provided with a battery-powered encoder that collects meter readings and periodically transmits those readings over a wireless network to a central station. The power limitations imposed by the need for the encoder to be battery-powered and by regulations governing radio transmissions effectively prevent direct radio transmissions to the central station. Instead, wireless meter-reading systems typically employ a layered network of overlapping intermediate receiving stations that receive transmissions from a group of meter encoders and forward those messages to the next-higher layer in the network as described, for example, in U.S. Pat. No. 5,056,107. These types of layered wireless-transmission networks allow for the use of lower power, unlicensed wireless transmitters for the potentially thousands of end-point encoder transmitters that must be deployed as part of a utility-meter-reading system for a large metropolitan area.
In 1985, as an attempt to stimulate the production and use of wireless-network products, the FCC modified Part 15 of the radio spectrum regulation, which governs unlicensed devices. The modification authorized wireless-network products to operate in the industrial, scientific, and medical (ISM) bands using spread-spectrum modulation. The ISM frequencies that may be used include 902 to 928 MHz, 2.4 to 2.4835 GHz, and 5.725 to 5.850 GHz. The FCC allows users to operate spread-spectrum wireless products, such as utility-metering systems, without obtaining FCC licenses if the products meet certain requirements. This deregulation of the frequency spectrum eliminates the need for the user organizations to perform costly and time-consuming frequency-planning to coordinate radio installations that avoid interference with existing radio systems.
Spread-spectrum modulators use one of two methods to spread the signal over a wider area. The first method is that of direct-sequence spread-spectrum (DSSS) while the second is frequency-hopping spread-spectrum (FHSS). DSSS combines a data signal at the sending station with a higher data-rate bit sequence, sometimes called a “chipping code” or “processing gain.” A high processing gain increases the signal's resistance to interference. FHSS, on the other hand, relies on the distribution of a data signal randomly hopped across a number of defined-frequency channels to avoid interference.
FHSS operates by taking the data signal and modulating it with a carrier signal that hops from frequency to frequency as a function of time over a wide band of frequencies. With FHSS, the carrier frequency changes periodically. The frequency-hopping technique reduces interference because an interfering signal from a narrowband system will only affect the spread-spectrum signal if both are transmitting at the same frequency and at the same time. Thus, the aggregate interference will be very low, resulting in little or no bit errors.
In the frequency-hopping systems described above, interference in the ISM band from various sources, such as geographical obstructions, unlicensed radios, portable phones, and the like, decrease the probability that transmissions will be received. If transmissions could be directed to clear channels and away from noisy ones, the probability of successful reception would be increased, thereby enhancing system efficiency.